H pipe - Lower end torque/power, deeper sound. The pulses will "run" into each other in the pipe. Helping reducing resonance. Think old school V8.
X pipe - High end gains. More horsepower overall. The pulses will "merge" better in the pipe. Causing a scavenging effect at higher RPMs. Think raspy newer high performance engines.
You can watch some shitstangs rev and compare x vs h in this video:
X pipe vs H pipe REVS- Mustang GT - YouTube
As for pipe size. 3". Don't even think of 2.5 at that kinda power. You'll choke it for sure. 2.5" is for around 400 horse. Read this:
http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/ The math holds up. 3" would be great on your setup.
